The generation and survival of all organisms depends on the faithful execution of cell division. A complete understanding of the regulation and controls over cell division is critical to elucidate the mechanisms that govern self-renewal, proliferation and development. A key event in the cell cycle is the precise partitioning of every pair of duplicated chromosomes to daughter cells. Defects in segregation lead to aneuploidy, the state where entire chromosomes are gained or loss. Aneuploidy is a hallmark of most tumor cells and has been postulated to be a major factor in the evolution of cancer. It is also the leading cause of spontaneous miscarriages and hereditary birth defects in humans. The proposed work will lead to an understanding of the mechanisms that ensure accurate chromosome segregation and thus maintain genomic stability and prevent human disease. Chromosome segregation requires forces generated by spindle microtubules that are translated into chromosome movement through interactions with kinetochores, the highly conserved structures that assemble onto centromeric chromatin. Accurate segregation requires kinetochores to maintain load- bearing attachments to the ends of microtubules that are continually growing and shrinking. Kinetochores must also biorient and attach to microtubules from opposite poles. When there is a defect in biorientation, error correction systems destabilize improper attachments. In the next funding period, this proposal will address outstanding issues in the field about the mechanisms that mediate kinetochore-microtubule attachment and the regulation over kinetochore biorientation. Together, these studies will be a means toward understanding the fundamental mechanisms of segregation in all eukaryotes.